1. Field of the Invention
The present invention relates to a process for producing a semiconductor device including a step for forming an electrode film on an insulating film. More particularly, the invention relates to a process for producing a semiconductor device enabling the insulating film to be made thinner.
2. Description of Related Art
The degree of miniaturization accompanying the scaling down of VLSI-MOS devices is limited only by the advances made in photolithography and other miniaturization techniques and has accordingly proceeded at a tremendous pace. There is said to be no limit to the reduction of size of transistors down to 0.1 .mu.m gates.
Accordingly, the gate insulating film is being made increasingly thinner with each generation. In 0.35 .mu.m rule devices, use is being made of gate insulating films of a thickness of about 10 nm. A double-layer film of a polycrystalline silicon film and tungsten silicide film (WSi.sub.x, wherein x is 2.2 to 2.8), that is, a so-called "polycide film", is generally superposed on the gate insulating film as a gate electrode film. It is known that when forming a tungsten silicide (WSi.sub.x) film, fluorine enters into the tungsten silicide (WSi.sub.x) film due to the use of silane gas and WF.sub.6 gas.
In this gate insulating film, however, it has also been known that if no special treatment is applied and the thickness is increased by natural oxidation by about 1 to 2 nm, an increase occurs in the thickness due to the enhanced oxidation based on the fluorine in the tungsten silicide film and it therefore is not possible to accurately control the thickness of the gate insulating film.
For example, as shown in FIG. 5A, if a sample prepared by depositing a polycrystalline silicon film on a silicon oxide film and then depositing a tungsten silicide (WSi.sub.x) film on it using SiH.sub.4 +WF.sub.6 is measured for the concentration of fluorine just after deposition by a secondary ion mass spectrometer (SIMS), it is found that the distribution in the tungsten silicide (WSi.sub.x) film is uniform at a high concentration. If the sample is heat treated at a high temperature of 950.degree. C. and then analyzed by a secondary ion mass spectrometer (SIMS), then, as shown in FIG. 6A, it is found that fluorine is taken in at the interface between the silicon oxide film and the polycrystalline silicon film. This fluorine causes enhanced oxidation of the silicon oxide film.
It was also known that if a high concentration of fluorine is contained in the gate electrode film (1.times.10.sup.21 atoms/cm.sup.2), the high temperature (700.degree. C.) annealing treatment used for lowering the resistivity, performed after the formation of the gate electrode film, causes the fluorine to rapidly escape as a gas and causes peeling of the gate electrode film by cracking the film.
Along with the miniaturization of devices, film-forming equipment has become larger in size as well. Film-forming apparatuses which form films at speeds 10 times that of the prior art have also been developed. If a tungsten silicide (WSi.sub.x) film is formed by such a 10-times faster film-forming apparatus, then two to three times the concentration of fluorine ends up being contained in the gate electrode film compared with the prior art and there is accordingly a greater increase of the thickness of the gate insulating film (growth of lower oxide layer) and a greater chance of peeling, so improvement has been required.
Inventors of the present application proposed to enable reduction of the concentration of fluorine in the tungsten silicide (WSi.sub.x) film to 1/2 to 1/10 that of the prior art by formation of the tungsten silicide (WSi.sub.x) film by a dichlorosilane (DCS)+WF.sub.6 gas and a film-forming temperature of 650.degree. C. (fall of 1988, Journal of the Japanese Society of Applied Physics, p. 616). We found that by using this tungsten silicide (WSi.sub.x) film for a polycide structure gate electrode film, it is possible to suppress the enhanced oxidation by fluorine and as a result possible to suppress the reduction in capacitance of the gate insulating film, prevent a reduction in the voltage resistance, and thereby prevent a reduction in reliability.
For example, as shown in FIG. 5B, if a sample prepared by depositing a polycrystalline silicon film on a silicon oxide film and then depositing a tungsten silicide (WSi.sub.x) film on it using a dichlorosilane (DCS)+WF.sub.6 gas is measured for the concentration of fluorine just after deposition by a secondary ion mass spectrometer (SIMS), it is found that the concentration of fluorine in the tungsten silicide (WSi.sub.x) film falls, as shown in FIG. 5B, compared with the case shown in FIG. 5A. Further, the results of analysis by a secondary ion mass spectrometer (SIMS) of a sample heat treated at a high temperature of 950.degree. C. are shown in FIG. 6B. As shown in FIG. 6B, compared with the case shown in FIG. 6A, the amount of fluorine taken into the silicon oxide film is also reduced.
In this method, however, there is a high temperature reliance and vacuum reliance at the time of film formation and the reproducibility and uniformity become poorer as compared to the case of formation of the tungsten silicide (WSi.sub.x) film using monosilane, so use of this for actual processes for production becomes difficult.
On the other hand, a process for production of a tungsten silicide (WSi.sub.x) film using a conventional silane type gas in which the film-forming temperature is reduced to about 360.degree. C. to 450.degree. C. to try to reduce the residual fluorine has been prepared, but it is not possible to sufficiently control the gas phase reaction and therefore it is difficult to obtain a tungsten silicide (WSi.sub.x) film at a practical manufacturing yield.
Further, proposal has been made to raise the temperature of the substrate before the film formation and then form a tungsten silicide (WSi.sub.x) film, but practical film-formation conditions have not been obtained.
For miniature gate electrodes, it has been proposed to eliminate these problems by making the gate electrodes of just a polycrystalline silicon film, but this ends up running counter to the miniature gates, for which high speed operation is desired, so that the number of interconnection layers would increase and the manufacturing costs rise.
At the present time, gate insulating films are becoming thinner than 8 nm and therefore there are problems in reliability of the voltage resistance.